The present invention relates generally to vehicle driveline systems, and more particularly, to an axle assembly having a worm gearset and a differential.
Differentials are used in various vehicular driveline applications for transferring rotary power from an input to a pair of outputs while permitting speed differentiation between the outputs. For example, in rear-wheel drive (RWD) vehicles, the rear axle assembly includes a final drive differential unit having a carrier and a gearset connecting the carrier to left and right axle shafts. As is conventional, a hypoid gearset is used to transfer power from the propshaft to the carrier. The hypoid gearset includes a bevel-type ring gear secured to the carrier and a beveled meshed with the ring gear and driven by the propshaft. The size of the ring gear and the number of teeth associated with both the ring gear and pinion are selected to provide a predetermined speed reduction between the propshaft and the carrier. However, the size of the ring gear greatly dictates the packaging requirements for the differential housing of the axle assembly.
Worm gearsets, otherwise referred to as worm/worm gear transmissions, are known in the mechanical power transmission field. The worm gear is driven by the rotation of the worm with which it meshes. The rotational speed of the associated shaft of the worm gear is a function of the number of teeth on the worm gear and the number of threads on the worm. The worm may be single or multiple threaded. Conventional worm/worm gear transmissions have a worm gear with at least 24 teeth. In particular, the American National Standard xe2x80x9cDesign of Industrial Double-Enveloping Wormgearsxe2x80x9d (ANSI/AGMA-6030-C87) recommends 24 as the minimum number of gear teeth. Furthermore, the enveloping angle of known worm gears for one revolution of the thread of the worm is not more than 5 degrees. Moreover, standard enveloping worm/worm gear transmissions have been used in drive axle only for ratios of five and more (U.S. Pat. No. 1,980,237) Due to such high ratios, it has been considered impractical to use the worm gear as the driven member and the worm as the driving member to transfer power from the worm gear to the worm in drive axle applications. In U.S. Pat. No. 6,098,480 tooth surface of a worm gear has a first worm gear pressure angle and a second worm gear pressure angle. The first worm gear pressure angle is twice as large as the second worm gear pressure angle. Worm gear with this profile has high contact stress and very poor lubrication condition.
Accordingly, it is an object of the present invention to provide a drive axle assembly equipped with a differential and a worm gearset that is smaller than standard differential and drive axle assemblies. Since enveloping worm has a thread less than one revolution positioned in a mesh with the worm gear it creates suction for better lubrication. Suction becomes likely due to asymmetrical profile of the enveloping worm. (Standard enveloping worm always has a symmetrical design). Combination of enveloping worm having the thread less than one revolution along with housing leads to production of cooling liquid (oil) stream. Enveloping worm works like a screw pump. The housing helps to separate low-pressure oil from high-pressure oil, which increases liquid flow through teeth mesh.
It is a further object of the present invention to provide a drive axle assembly equipped with a differential that is lighter in weight and quieter in operation compared to conventional drive axle.
These and other objects of the present invention are obtained by providing a drive axle assembly with a unique worm-type input gearset. In particular, a drive shaft having an enveloping worm mounted thereto is meshed with a worm gear, which is fixed to an input member of the differential, where worm gear teeth has a tooth surface generated by a profile of the enveloping worm.
Preferably, the worm gear is secured to a carrier of the differential. The carrier of the differential supports a gearset including pinion gears, which are meshed with side gears, with each side gear secured to corresponding left and right axle shafts.
Enveloping worm/worm gear transmissions with a worm gear having less than twenty-four teeth have not been commercially used because it was believed impossible to build such a transmission due to the need to undercut the root of the worm gear tooth. Thus, those skilled in the art did not consider enveloping type worm gears with less than twenty-four teeth to be feasible for commercial applications. In contrast, the enveloping worm/worm gear transmissions of the present invention utilize a worm gear without undercut gear teeth because of a greater enveloping angle for one revolution of the worm thread.
With less than twenty-four gear teeth and a greater enveloping angle for one revolution of the thread, as compared to prior enveloping worm/worm gear transmissions, the minimum ratio for one thread could be reduced to two. In contrast, prior enveloping worm/worm gear transmissions had a minimum ratio of twenty-four for one thread of the worm and a ratio of five for five threads of the worm. The efficiency of the new worm/worm gear transmission is even greater than that of well-known hypoid gearsets that are used in low ratio right angle drives. Thus, the present invention can replace hypoid or bevel gearing in many applications by reason of the low ratio. In addition, this new worm/worm gear transmission is able to back drive by transmitting torque from the worm gear to the worm. For the same size, this invention has more than twice the capacity of traditional hypoid gearing.
In the present invention, the worm can be half or less than half of a split worm, which can have only one supporting shaft. Further, the worm gear can be half or less than half of a split worm gear, which may have a supporting shaft on only one side of the worm gear. Using only half or less than a half of the split worm gear or worm enables easier assembling of the worm with the worm gear. Half or less than a half of the worm indicates that worm thread has less than one revolution.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood however that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.